Flexible storage tanks (sometimes referred to as flexitanks) are large bladders used to transport liquids or flowable materials, including highly viscous materials. The bladders are typically constructed of one or more layers or plies of a flexible material (such as two layers of polyethylene (PE) materials, 4-40 mills in thickness), forming an interior water proof (or “fluid proof”) portion in which fluids are stored for transport in inter model containers. Flexible means the material can be folded upon itself without fracturing. An example of a prior art flexitank is shown in U.S. Pat. No. 4,468,812. Flexitanks have several advantages—maximum use of space (as opposed, for instance, to drum transport), ease of loading and unloading. They can be made from food-grade materials, and do not have to be cleaned after use, as they are disposable.
A filled bladder is supported by a metal transport container, such as a standard 20 foot sea or railcar transport container, generally referred to as a Sealand Container or a modular transport container. A bulkhead usually is installed in the transport container to keep a filled flexitank from exerting pressure on the container's doors. A typical size for an unfilled flexitank, for use in a 20′ long Sealand container is 23.2 feet long by 12.8 feet wide. For reference, assuming a bladder having a length that is greater than its width, the long dimensioned length will be termed “sides” or S while the shorter dimensioned width will be termed “ends” or E. A bladder also has a top portion “T” and a bottom portion “B”, referenced in orientation of a filled flexitank (e.g., the bottom portion B is in contact with and supported by the transport container floor.)
The flexitank includes at least one sealable opening into the interior, generally sealed with a valve. The valve is used to fill and discharge the bag. The flexitank may have additional sealable openings as needed for particular applications (such as a vent). The valve may be on the top of the bag, or on the end of the bag, and is positioned on the bag for ease of access for filling and discharging of the flexitank.
To fill a flexitank, the empty bladder is positioned in the interior of a transport container. The bottom (and possibly a portion of the sides) of the container may be lined, for instance, with corrugated paper, boards or other material to protect the flexitank from abrasion induced damage. A fill line is coupled to the valve on the flexitank. If a bulkhead is used, the valve should be accessible through the bulkhead. Product is then pumped into the flexitank, and the flow is metered. Once the desired capacity is reached (usually the rated capacity of the flexitank, for instance, 5000 gallons), the valve is closed and the fill line or hose is removed. A filled flexitank has a known circumference.
During transport, product inside the bladder interior will shift in response to external conditions. In particular, on an ocean going vessel, wave action will translate to fluid movement within the bladder, and the fluids within the flexitank also exhibit wave action. Because the bladder is constructed of pliable elastomeric materials, the exterior of the bladder will stretch and deform in response to fluid movement. This can result in elongation of the bladder, change in circumference, and possible damage to the flexitank and to the transport container.
To reduce stresses on a flexitank, additional layers of material can be added, such as incorporating a non-woven geotextile polypropylene in the construction of the flexitank. See U.S. Pat. No. 6,626,312, hereby incorporated by reference. Another suggested modification has been to strap the bladder itself to the transport container, such as shown in U.S. Pat. No. 6,626,312.